Metallic weld studs are typically solid, non-compressible bodies which are formed by standard fastener stamping methods such as being cold headed from rod materials. Such weld studs are welded to a component surface by using a known welding device which provides energy through the weld stud to melt both a circular sacrificial weldment element, as well as a portion of the component base material. The weld stud is fixed to the base material due to the commingling and cooling of the liquefied metals. The welding device, in particular a weld gun, grasps the weld stud using clamping jaws in a region between a shoulder of the stud and the end of the face to be welded. Each weld stud is placed into the mouth of weld gun one at a time immediately prior to welding. The size, weight, and configuration of these studs prevent their use in blow fed weld guns.
Increased fuel economy demands within transportation industries have led to the use of thinner and thinner gauge materials. In particular, metallic composite materials and particularly aluminum composite materials with a polymer core have effectively been used as surface or skin materials for various structures. These very thin metallic laminates are typically coupled or fastened to a support structure by being fastened to the support structure on one side of the laminate. To this end, it is known to fasten an ordinary weld stud to the laminate structure to facilitate the coupling of the stud to the structure.
Traditionally, weld studs burn through at least the first layer of the thin laminate material. The weldment provides a stress riser which significantly weakens the strength of weld stud laminate interface. Repeated loading of the weld stud leads to plastic deformation in the coupling interface and premature failure of the stud to laminate interface.
Known studs have a head and shank connected to the head. The head of a stud is welded to the sheet metal of a vehicle body. The welding operation takes place according to the known method of drawn-arc welding. In this method, the stud is brought into contact with the sheet metal, the welding current is then switched on and the stud is drawn off the sheet metal so that an arc is formed between the stud and the sheet metal. While the arc is burning, part of the stud head and part of the sheet metal melt. When a sufficient amount of molten metal has been generated, the stud is driven into the melting. The welding operation as such can be influenced by several parameters. The influence of the individual parameters has various effects on susceptibility to error when welding studs with the drawn-arc method.
For the purpose of arc initiation, the head geometry of the stud must be designed accordingly. In this context, studs having a head with a conical tip are known in the art. Studs with a substantially flat head/front end are further known, with an ignition tip formed in the center of the front end. Studs with a flat front end are further known in the art. The threaded stud is welded onto a metal sheet of the body in so-called short-time arc welding. Short-time arc welding is also known as stud welding, where a metal stud (threaded stud) is positioned so as to contact the sheet metal of the body. A pilot current is then turned on and the metal stud is again slightly lifted off from the sheet metal of the body. At the same time, an arc is drawn. Then, a welding current is turned on so that the facing surfaces of metal stud and body sheet metal are fused. The metal stud is then again lowered onto the sheet metal of the body so that the melts combine. The welding current is turned off and the whole fused mass solidifies.
A lock nut is then typically screwed onto the stud, thus projecting from the sheet metal of the body. The nut acts to fix the member to the sheet metal. As a rule, the lock nut is made of synthetic material. The stud may be a coarse-pitch threaded stud or a fine-pitch threaded stud. A matching thread is provided on the lock nut. In the case of a coarse-pitch thread, it is alternatively possible that only one hole is provided on the lock nut. The coarse-pitch thread then cuts a corresponding counter-thread into the hole.
Fractures of threaded studs and of metal body sheet occur in undefined fashion. It is hard to establish what the reason for the failure was. In addition, reworking of the fractured sheet metal of a car body requires a considerably greater expenditure than reworking in the case of a fractured stud. In a fracture of the stud, a new stud can be welded at the same spot, without the strength of the sheet metal suffering.
Against this background, the problem underlying the invention is to provide an improved fastening system of generic type which, in particular, requires little reworking in the situation when the coupling of a nut to the welded stud structure fails.